When supplying power to a load via an AC power supply system, it is helpful to know whether the AC power supply is wired correctly (e.g., whether the line, neutral, and ground terminals of the supply are properly connected). Plugs of power-sensitive electrical/electronic equipment have standardized configurations to mate with AC power supply sockets, such that the line and neutral terminals of the 120 Vrms supply system can be connected only to the line and neutral leads of the equipment, respectively. However, it is not uncommon for the line and neutral conductors of an AC power outlet to be wired in a configuration that is the reverse of the indicated arrangement, with 120 Vrms being supplied at the neutral terminal of the power outlet instead of the line terminal of the power outlet. Under such interchanged conditions, the line-to-ground and neutral-to-ground common mode paths are effectively reversed, which, if not properly accounted for in the design of the power filter circuit, creates a risk of excessive leakage current or inadequate EMI protection, as well as causing operational difficulties in the connected equipment. Consequently, devices that alert a user to a reversed wiring condition in the AC supply system can be useful in power filter circuits. However, such devices do not automatically correct reversed wiring conditions. Thus, more complicated or expensive power filter circuit designs are typically required to account for the possibility of a wiring fault condition, such as a reversed line and neutral arrangement.
One approach to designing AC power filter circuits to address potential wiring faults is simply to assume that the 120 Vrms signal may be present on either the line or neutral conductor of the AC power supply. Most prior art AC power filtering circuits generally use an impedance network relying largely on capacitors to suppress common mode interference. Because of the possibility of having 120 Vrms between any two conductors, these devices must use relatively small capacitors between all of the conductors, including the neutral and ground conductors, to meet UL leakage current requirements. Further, these power filter circuits conventionally include clamping devices, such as varistors, connected between each pair of conductors (e.g., in parallel with the capacitors). Below their rated voltage, these clamping devices are non-conductive, thereby presenting an open circuit. Above their rated voltage, these clamping devices conduct, thereby creating a short circuit designed to prevent transients above the rated voltage from reaching the connected electronic equipment. Once again, however, because of the possibility of having 120 Vrms between any two conductors, all of the clamping devices, including the clamping device connected between the neutral and ground conductors, must be rated for more than 120 Vrms. This results in an arrangement that can, at best, suppress neutral-to-ground disturbances to about 200 V and, when subject to the high transient energy levels that often occur on the 120 Vrms supply system, may allow up to 500 V or more to reach connected equipment. For sensitive equipment, this can lead to disruption or damage. Thus, because of the risk of a reversed line-neutral condition, these devices provide relatively poor common mode suppression of low frequency interference between the neutral and ground conductors and relatively poor common mode suppression of transients between the neutral and ground conductors.
Another proposed solution to addressing reversed wiring conditions (i.e., interchanged line and neutral terminals) is to use a switching circuit that includes a double pole, double throw (DPDT) relay adapted to connect the input (AC supply) and output (load) line and neutral conductors in accordance with the output of a wiring fault sensing circuit. When the wiring fault sensing circuit detects that the supply system circuit has all three conductors connected and the AC signal (120 Vrms) is present on the line conductor, the DPDT relay respectively connects the input line and neutral conductors to the output line and neutral conductors. Conversely, when the wiring fault sensing circuit detects that the line and neutral conductors of the AC power supply system have been interchanged, the DPDT relay switches states to connect the input line conductor to the output neutral conductor and to connect the input neutral conductor to the output line conductor, such that the 120 Vrms appears at the output line conductor. Accordingly, the correctly wired signal is passed to the connected equipment.
Theoretically, the above-described DPDT circuit would seem to address many of the noted existing problems. However, available DPDT relays are not designed to handle the current requirements of a power filter circuit. In particular, no commercially available DPDT can handle 20 amps in both the normally open and normally closed positions. For example, many DPDT relays are rated for 20 amps for a normally open circuit, but rated for only 10 amps for a normally closed circuit. To employ a DPDT relay in the manner proposed, a 20 amp rating would be required for both the normally open and normally closed cases. Thus, commercially available and affordable DPDT relays do not provide a workable solution for correcting reversed wiring conditions.
Another contemplated solution is to use two double pole, single throw (DPST) relays controlled by a wiring fault sensing circuit. However, a coordination problem between the relays arises when the switching signal is applied to the relay. In theory, when the switching signal is applied to a relay, the relay cleanly switches from one position to another. In practice, however, there is a delay time between signal application and actual relay switching, and timing differences between the two DPST relays can result in brief short circuits that may potentially damage equipment. Further, the strong springs used in relays designed to handle substantial currents may cause the relays to experience some mechanical bounce at the time of switching, such that the relays may briefly switch back and forth between contact positions, resulting in temporary short circuit conditions. Thus, DPST relays do not provide a consistent, reliable solution to correcting wiring faults of AC power supply systems.
Consequently, there remains a need for a wiring fault correction circuit that corrects a reverse AC supply line-neutral wiring condition consistently, cost-effectively, and in compliance with UL requirements. Additionally, it is desirable that such a wiring fault correction circuit be capable of being included in a cost-effective power filter circuit that meets UL requirements under reverse supply wiring conditions.